The field of the invention is gyromagnetic resonance spectroscopy, and particularly, nuclear magnetic resonance (NMR) techniques for measuring flow.
Gyromagnetic resonance spectroscopy is conducted to study nuclei that have magnetic moments and electrons which are in a paramagnetic state. The former is referred to in the art as nuclear magnetic resonance (NMR), and the latter is referred to as paramagnetic resonance (EPR) or electron spin resonance (ESR). There are other forms of gyromagnetic spectroscopy that are practiced less frequently, but are also included in the field of this invention.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the paramagnetic nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant of the nucleus).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.z) the individual magnetic moments of the paramagnetic nuclei in the tissue attempt to align with this field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M is produced in the direction of the polarizing field but the randomly oriented components in the perpendicular plane (x-y plane) cancel one another. If, however, the substance, or tissue, is irradiated with a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, can be rotated into the x-y plane to produce a net transverse magnetic moment M.sub.1 which is rotating in the x-y plane at the Larmor frequency. The degree to which the rotation of M.sub.z into an M.sub.1 component is achieved, and hence, the magnitude and the direction of the net magnetic moment (M=M.sub.z +M.sub.1) depends primarily on the length of time of the applied excitation field B.sub.1.
The practical value of this gyromagnetic phenomena resides in the radio frequency signal which is emitted as a result of the net transverse magnetic moment M.sub.1. One commonly used technique, referred to in the art as a "pulsed NMR measurement", applies the excitation field B.sub.1 for a short interval, and then receives the signal that is produced by the resulting transverse magnetic moment M.sub.1. Such pulsed NMR measurement cycles may be repeated many times to make the same measurement at different locations in the subject or to make different measurements using any of a number of preparative excitation techniques. For example, in my U.S. Pat. No. 4,516,075 I disclose how a bipolar magnetic field gradient may be applied between the excitation of the gyromagnetic material and the receipt of the subsequent radio frequency emission to "sensitize" the emitted signal to indicate the direction and magnitude of fluid flow. When combined with imaging techniques such as that referred to as "zeugmatography", two dimensional images indicating the magnitude of fluid flow may be produced using this pulsed NMR technique.
Another common NMR technique which has been employed to measure the flow of fluids applies a continuous excitation field B.sub.1 to the gyromagnetic material. A paper "The NMR Blood Flowmeter-Theory and History" by J. H. Battocletti et al., published in Medical Physics Vol. 8, No. 4, July/August, 1981, describes the theory and history of this effort. Such "CW" techniques employ special NMR apparatus with coils arranged to magnetize a sample of the fluid "upstream" of the coils which are employed to sense the emission signal. The physical distance between this "tagging" coil and the sensing coil is known, and the level of the emission signal provides velocity information in the direction of fluid flow. An apparatus for measuring blood flow in this manner and producing two dimensional images is disclosed in U.S. Pat. No. 4,613,818 entitled "Nuclear Magnetic Resonance Blood Flowmeter".
While CW NMR techniques are effective for measuring pulsatile flow, or for measuring continuous flow in well defined structures such as pipes, they have not been entirely satisfactory for measuring blood flow which has both continuous and pulsatile components and which is contained in irregular shaped vessels.